Manage your subscription

Mining enters the third dimension

By Bob Johnstone

GOLD-SEEKING geologists in Western Australia are trying to unravel
what to them is the world’s greatest detective story. If they can work out how
rocks in the southern part of the state were formed, they could well stumble
across a mother lode as rich as the 70 million ounce deposits of the Golden Mile
in Kalgoorlie. But the trail is cold—very cold—and clues are scant.
The rocks they are investigating were laid down 2.6 billion years ago, hardly a
fresh scent for gold-seekers. Now the mystery could be solved with the help of
3D modelling technology and an understanding of the recent geological history of
the Indonesian island of Sulawesi.

The key character in the story is Nick Archibald, a structural geologist from
Perth. Frustrated by a lack of software to allow the application of 3D imaging
to geology in general and mining exploration in particular, Archibald and his
colleagues formed a company called Fractal Graphics.

Working in partnership with the Australian Geodynamics Cooperative Research
Centre in Nedlands in Western Australia, Fractal Graphics has recently completed
a 3D model of an area called the Menzies-Norseman belt, which runs roughly
between the towns of Menzies and Norseman in Western Australia, about 600
kilometres east of Perth. The belt encompasses the Eastern Goldfields, which is
the area around Kalgoorlie.

By digitising sets of existing data such as drill hole logs, Archibald and
his colleagues have produced a model of the upper part of the Earth’s crust to a
depth of 10 kilometres. The model, at a scale of 1:100,000, covers about 30 000
square kilometres. The AGCRC claims that this is the first time such a
large-scale model at this resolution has been attempted. (Interested readers can
see for themselves at www.agcrc.csiro.au/projects/3021CO/results.htm)

Advertisement

But even with such a powerful tool, figuring out which geological structures
contain gold is not easy. A lot can happen in 2.6 billion years, when the gold
would have been deposited. The rocks have undergone extensive weathering and
deposition of sand and other minerals. This is where Sulawesi enters the
plot.

While developing the Menzies-Norseman model, Archibald was also working as a
consultant for exploration companies in Sulawesi. Drawing on information gleaned
from oil company surveys of the area, he was able to make a comparison between
cross-sections of Sulawesi and the Eastern Goldfields. There were obvious
differences, but also a number of important similarities. “The rocks in Sulawesi
are younger and you can see them at the surface,” Archibald said. “They haven’t
been subjected to the weathering and erosion you see in Kalgoorlie.”

Nevertheless, these same rocks hold clues to what happened so long ago in WA.
Sulawesi, about 800 kilometres northwest of Australia across the Timor Sea, sits
above the junction of two tectonic plates, one of which is being pushed up, the
other sinking down. The area is volcanically active. Seismic processes are
bringing to the surface granites and green stones, the same gold bearing rocks
found in WA.

Archibald’s colleague, Graham Price, the director of the AGCRC, says that a
young and active area like Sulawesi can show how gold-carrying fluids are formed
deep within the Earth and where they penetrate into the upper crust. He refers
to Sulawesi as a field laboratory where modern tectonic movements will provide
insights into the evolution of the Kalgoorlie region. The geologists believe
that this knowledge, when combined with the computer-generated 3D models,
will help them to predict the location of new deposits.

Statistics give the prospectors hope. Plotting size versus numbers of
deposits reveals a gap between the Golden Mile, which is large and rich, and the
next biggest Australian deposits, which are much smaller. Natural distribution
suggests that there should be something in between just waiting to be
discovered.

But many of the smaller mines in WA are petering out. The gold mining
industry finds itself caught between a rock and a hard place. As the quality of
deposits drops, mining is becoming more expensive. At the same time, the price
of gold continues to decline.

Today, mining is often only marginally profitable, a situation which makes it
increasingly difficult to attract investors. Exploring for new deposits remains a
highly risky business. And with prospectors mothballing their drill rigs, the
industry is desperately looking for new ways to improve the odds. Archibald and
his colleagues, with some help from Sulawesi, could have the answer.